Field of the Invention
The present invention relates to an optical tomography observation apparatus that observes a measurement target using interference of light.
Background Art
In recent years, an optical coherence tomography (OCT) that obtains an image of a surface structure or an inner structure of a measurement target by using interference of light has attracted attention. Since the OCT has no invasiveness to a human body, the application of the OCT to a medical field or a biological field has been especially expected, and an apparatus that forms an image such as an ocular fundus or a cornea has been put to practical use in an ophthalmological field. For example, as described in U.S. Patent Publication No. 2012-0300217, in the OCT, light from a light source is branched into two, that is, signal light applied to the measurement target and reference light reflected by a reference light mirror without being applied to the measurement target. The signal light reflected from the measurement target is combined with the reference light, and interference is caused in the combination light to obtain a signal.
The OCT is largely classified into a time domain OCT and a Fourier domain OCT according to a scanning method (hereinafter, referred to as a z-scan) in an optical axis direction of a measurement position. In the time domain OCT, the z-scan is performed by using a low coherence light source as a light source and scanning a reference light mirror, at the time of the measurement. Thus, only components which are included in the signal light and have the same optical path length as that of the reference light interfere, and envelope detection is performed on an obtained interference signal to demodulate a desired signal. Meanwhile, the Fourier domain OCT is classified into a wavelength scanning OCT and a spectral domain OCT. In the wavelength scanning OCT, the z-scan is performed by using a wavelength scanning light source capable of scanning a wavelength of emission light and scanning the wavelength at the time of the measurement, and Fourier transform is performed on wavelength dependence (interference spectra) of detected interfering light intensity to demodulate a desired signal. In the spectral domain OCT, the z-scan is performed by using a broad-bandwidth light source as a light source, spectrally separating generated interfering light by a spectrometer, and detecting interfering light intensity (interference spectra) for each wavelength component so as to correspond to the z-scan. A desired signal is demodulated by performing Fourier transform on the obtained interference spectra.
In general, when a living body is measured in the OCT, reflection light from inside of a measurement target is extremely smaller than surface reflection light of the measurement target (or reflection light from an interface between the measurement target and a measurement target holding section such as a cover glass or a culture container of a cell). For example, a case where a cell within a culture container filled with a culture medium is measured will be described as shown in FIG. 1. An index of refraction of a typical culture container (made of polystyrene) is approximately 1.59, an index of refraction of a cell is approximately 1.37, and reflectance of an interface between the culture container and the cell is estimated as a value of approximately 0.55% from the indices refraction. Meanwhile, when an index of refraction of the culture medium is approximately 1.33, reflectance of an interface between the cell and the culture medium is approximately 0.022%. It is considered that reflection of an interface between different cells or reflectance in the cell are smaller values than the aforementioned values. Thus, the reflection light from inside of the measurement target may be buried in considerably intense surface reflection light, and, thus, it may be difficult to vividly visualize an inner structure near the surface of the measurement target.
In order to verify an influence of the surface reflection light, a measurement target in which two interfaces having a reflectance of 1% exist inside of a surface having a reflectance of 10% at a space of 5 um as shown in FIG. 2 is considered. A solid line of FIG. 3 represents an example of a signal waveform when an imaging signal is obtained along a z-scan axis shown in FIG. 2 by using an OCT apparatus having a vertical resolution of approximately 3 um. A peak of a second interface position can be clearly recognized. In contrast, a peak of a first interface position mostly disappears due to interference with a surface reflection light component, so that it is difficult to recognize presence of a first interface.
As means for suppressing the influence of the surface reflection, a method of subtracting the surface reflection light component from the imaging signal is considered.
FIG. 4 shows a result obtained by subtracting the surface reflection signal component (dashed line) from the imaging signal (solid line) shown in FIG. 3. It can be seen that a peak appears in a position (depth of 2 um) where there is no interface, a peak position of the first interface is shifted from an original depth of 5 um to a depth of approximately 6 um, and a signal that accurately reflects a structure of the measurement target is not obtained. As mentioned above, in the method of simply subtracting the surface reflection signal component from the imaging signal, since a light interference effect is not considered, it is difficult to sufficiently suppress the influence of the surface reflection light, and, thus, it is difficult to accurately capture the structure of the measurement target.
As stated above, in the OCT apparatus of the related art, it may be difficult to vividly visualize the structure near the surface due to the influence of the intense surface reflection light of the measurement target.